Bleeding (hemorrhage) causes 30-50% of all trauma-associated deaths. Permissive hypotension or hypotensive resuscitation in pre-hospital and critical care environments helps prevent re-bleeding from severe injuries. Restoration of blood pressure after trauma or uncontrolled hemorrhage can actually be detrimental by causing more bleeding. Maintenance of low but critical level of systolic blood pressure can reduce bleeding and help preserve a minimal level of tissue perfusion. A minimum level of systolic blood pressure (70-90 mmHg) is needed to maintain a critical level of coronary and cerebral perfusion pressure to prevent cardiovascular or cerebral collapse. When brain injury is present, minimum values may need to be higher. In order to safely practice permissive hypotension, continuous non-invasive blood pressure monitoring is essential in maintaining permissive hypotension, but is challenging in the pre-hospital setting. Invasive blood pressure monitoring while allowing for continuous monitoring is not practical. It is also associated with risk such as infection. Even in the hospital setting, invasive monitoring is not always practical.
Photoplethsmography (PPG) (pulse oximetry) provides a noninvasive measure of arterial hemoglobin oxygen saturation. The technology relies on pulsatile flow to the finger or other part of an extremity. Very little pulsatile flow is required to produce this reading. Other technologies such as passive piezoelectric sensors are capable of picking up pulsatile flow as well but as opposed to the PPG which senses flow, piezoelectric sensors measure the mechanical expansion and contraction of the arterial vessel wall in response to pulsatile flow. Applanation tonometry, Doppler signal, bioimpedence sensor, or light spectroscopy are suitable alternatives for pulsatile or piezoelectric sensors. This invention uses a cuff or other pressure-producing device to make a measurement necessary to provide health care provider scheduled knowledge of both the current systolic pressure as well as when systolic blood pressure has been reduced to a critical threshold. The invention can be used in conjunction with other devices to create a closed loop feedback system to maintain critical blood pressure and other perfusion values. The PPG, piezoelectric sensors, applanation tonometry, Doppler signal, or light spectroscopy sensors may be disposed either distal to the cuff or other pressure producing component, under the pressure producing component, or both distal and under the pressure producing component.
It is well known that the use of an inflatable blood pressure cuff when coupled with a distal PPG can be used to determine systolic blood pressure. Systolic blood pressure can be determined by inflating the blood pressure cuff to a level that causes the disappearance of the PPG signal. However, continuous measurement of systolic blood pressure by this manner is not advised as ischemia would result. Furthermore it is not necessary to do this to practice permissive hypotension. Current automated oscilliometic noninvasive blood pressure devices are ill suited to dynamically monitor blood pressure for permissive hypotension.